American Lion, or Giant Jaguar? – In Search of Panthera atrox

The Page Museum at the La Brea Tar Pits is a wonderful place. I don’t mean wonderful in the overused, everyday sense of “That pizza was wonderful”, or “If you could hand in your TPS report before you leave the office, that would be wonderful.” I mean that the museum is literally a place that fills my fossil-loving heart with wonder. Even though I had seen photos of the exhibit before, I was still stunned by the glass case containing the dark brown skulls of over 350 dire wolves, and I was delighted to get a brief tour of the fossil-filled boxes that are currently being excavated behind the building.

But there was one thing that hooked into my brain and wouldn’t let go during my visit to the Page. Among the museum’s exhibits were composite skeletons of some of the charismatic Pleistocene mammals which perished in the asphalt seep that now sits in the middle of Los Angeles. All the classic characters were represented – the Shasta ground sloth, Bison antiquus, the deep-snouted bear, the sabercat Smilodon, the Columbian mammoth, and others. I had read about these animals before and seen other skeletons of their kind elsewhere, but the label on one of them puzzled me. For as long as I could remember, the big cat Panthera atrox was commonly known as the “American lion” – a now-eliminated branch of the famous cats which long-ago reached North America. The plaque in front of the formidable pantherine’s skeleton called it something else – Naegele’s giant jaguar. Since when had America’s own lion been transformed into a burly jaguar?

A pair of publications I picked up from the museum’s gift shop further catalyzed my confusion. In a revised, 2001 edition of paleontologist Chester Stock’s monograph Rancho La Brea: A Record of Pleistocene Life in California the fossil cat was cast as “lion-like”, though the brief passage also mentioned that Panthera atrox “has also been called a gigantic jaguar.” Not much help. The glossier, popular-audience pamphlet Rancho La Brea: Death Trap and Treasure Trove didn’t resolve the issue, either. A summary contributed by George T. Jefferson simultaneously identified the cat as both Naegele’s giant jaguar and a subspecies of lion (Panthera leo atrox). While treated and illustrated as a lion, Jefferson repeatedly called the animal a giant jaguar. Clearly, I had to dig deeper.

An illustration of the lower jaw of "Felis" atrox described by Leidy. From Leidy, 1852.

Paleontologists have been kicking around different ideas about the identity of Panthera atrox since the mid-19th century. On the basis of a partial lower jaw containing three molars and a broken canine, the Philadelphia polymath and naturalist Joseph Leidy called the fossil cat “Felis” atrox. He described the cat as “a species which much surpassed in size the recent Tiger and Lion, or the extinct Felis spelaea [“steppe lion”] of Europe.” The specific traits of the jaw resembled their counterparts in all three big cats – living and extinct – but Leidy ultimately categorized the felid as “an extinct species of American lion.”

Not everyone was agreed that Leidy’s cat – what we now call Panthera atrox – was really a lion. During the first half of the 20th century, especially, experts on fossil mammals such as the French paleontologist Marcellin Boule and his German colleague Max Hilzheimer noted that Panthera atrox appeared to exhibit a mosaic of features shared with both lions and tigers. This uncertainty was echoed by studies of some of the early fossils excavated from the La Brea asphalt seeps and described by paleontologists John Merriam and Chester Stock. In their major 1932 monograph on Panthera atrox, the two concluded that the skull of the fossil cat most closely resembled that of the jaguar, Panthera onca, a few specimens of which have also been found in the same deposits. The eminent American paleontologist George Gaylord Simpson agreed with the determination of Merriam and Stock in his own 1941 review of the Pleistocene’s large fossil felids, and he bemoaned the fact that Panthera atrox had become popularly known as the “American lion.” The cat was no lion, Simpson argued, just as saber-toothed cats were not truly tigers. Continued use of the outdated terminology would only further mislead an American public that had a hard enough time understanding family relationships among fossils as it was.

Of course, the interpretations of Boule, Hilzheimer, Merriam, Stock, and Simpson were not universally accepted, either. If anything, the image of Panthera atrox as a North American lion held out against the jaguar interpretation. The cat, as I first encountered it, was a huge lion which hunted alone or in pairs in the open landscapes of the Pleistocene west. Most museums I have visited with Panthera atrox mounts have followed this trend. But, in my search for more information, I found at that a more jaguar-like version of the cat was again proposed just two years ago by paleontologists John M. Harris – curator of the Page Museum – and Per Christiansen.

Christiansen and Harris focused on the cranium and jaws of Panthera atrox. More than any other part of the skeleton, the head of the cat has most often cited as evidence that Panthera atrox shared more in common with jaguars than lions or other big cats. In order to detect the affinities of the fossil cat, Christiansen and Harris compared a series of measurements from a sample of well-preserved Panthera atrox specimens from the La Brea deposits to the same measurements made from the skulls of tigers, lions, and jaguars kept at other museums. The sample, as Harris recounted in response to an email I sent about the study, included “78 tiger skulls, 176 lion skulls and 57 jaguar skulls, [and] the 14 P. atrox skulls from Rancho La Brea.”

As many paleontologists had determined before, the skulls of Panthera atrox exhibited a mosaic of features which did not entirely match any of the living species. Whereas the crania of the fossil cats were more lion-like than anything else, some aspects of their lower jaws appeared to be more tiger- or jaguar-like. When I asked Harris about which features particularly placed Panthera atrox closer to jaguars than to lions, he cited the “mandibular ramus, mandibular symphysis, fronto-maxillary suture, and shape of nasal opening.” The last of these features is self-explanatory, but, for those who have not taken osteology in a while, the other three are the large flange of the lower jaw which connects to the cranium, the fused portion of the lower jaws which meet at the midline, and a suture in the skull along the frontal and upper-jaw (maxillary) bones. The Panthera atrox skulls, Harris said, “were closer to lions or tigers in some other features but multivariate and other statistical analysis suggested that P. atrox did not group with lions and was closest to jaguars.”

Curiously, though, in the paper Christiansen and Harris pointed out that the crania of Panthera atrox most closely resembled those of lions and deviated most from the crania of jaguars. As they wrote in the paper, “Panthera atrox differs from the lion, jaguar, and tiger in many osteometric skull variables, most often from the jaguar (21) and least often from the lion (16).” Even in regard to the fossil cat’s lower jaw, the range of variation among the lions and other extant cats in the study resulted in some of the comparisons being murky. Panthera atrox was not precisely like a modern day lion, but, from what I could see, the data gave no clear sign that the cat could be accurately called a “giant jaguar”, either.

Nevertheless, Christiansen and Harris cited their results as support for the idea that Panthera atrox occupied an intermediate place between a subgroup of lions/leopards on the one hand and tigers/jaguars on the other. Rather than being “a kind of giant, North American lion,” the researchers hypothesized, the anatomy and reconstructed ecology of the animal hinted that Panthera atrox “appears to have been close to a type of giant jaguar” which eschewed the forest for more open hunting grounds. I asked Harris, in his estimation, what the cat would have looked like when alive. He replied:

P. atrox is appreciably larger than both jaguars and lions. It had proportionately longer legs than those of jaguars, which may have been an adaptation for running in more open habitat. If so, this might explain why jaguars and giant jaguars are seldom found at the same locality. They both occur at La Brea, where we have over 80 individuals of P. atrox but only one or two of P. onca. What the pelage of P. atrox looked like is open to conjecture. If, indeed, it was a more open habitat form it may have had an unspotted or only faintly spotted coat.

The different interpretation had more than cosmetic implications. Christiansen and Harris suggested that this different perspective indicated that Panthera atrox was not descended from prehistoric lions which crossed into North America via the Bering Land Bridge. Instead the researchers hypothesized that Panthera atrox speciated from an earlier form of jaguar around 150,000 years ago. In this scenario, there were never any true “American lions” at all. The closest thing would have been lions which lived in the vicinity of the Bering land bridge but did not travel further south.

I have to admit, I wasn’t sold on the idea that Panthera atrox was more of a jaguar than a lion. Perhaps the cat shared some lower jaw traits with jaguars and tigers, but the data from the crania, especially, indicated that the cat was more like a lion than anything else. And, after all, anatomy isn’t everything when it comes to Pleistocene mammals.

The skulls of Panthera atrox from La Brea may show some slight differences when held up to those of modern lions, but genetic comparisons have pinned down the fossil cat as a member of the lion lineage. That’s the fortunate thing about studies of creatures which died in the not-too-distant prehistoric past – genetic material can be collected, analyzed, and studied to further test ideas made on the basis of skeletal anatomy. In a study published the same year as that of Christiansen and Harris, zoologist Ross Barnett and colleagues reported that Panthera atrox formed a distinct genetic cluster among prehistoric lion populations which became genetically isolated around 340,000 years ago. Even more specifically, Barnett and co-authors noted “All late Pleistocene lion samples produced sequences that grouped strongly with modern lion data”, and this finding ruled out “any postulated link between [Panthera] atrox and jaguar.”

What Panthera atrox was, and how the cat made a living on the late Pleistocene landscape, lies between the common images of “American lion” and “giant jaguar.” Even Christiansen and Harris noted how lion-like the cat’s skull was, and the genetic data clearly place Panthera atrox within the lion lineage. Nevertheless, there are a few curious things about the La Brea specimens that may indicate that these beasts did not act just like modern lions.

Only about 80 individuals of Panthera atrox are known from La Brea, which is quite low compared to the thousands of dire wolf and Smilodon individuals. Since dire wolves are thought to have been pack hunters, their prevalence in the asphalt seeps makes sense, and the high number of Smilodon individuals – along with other lines of evidence – has been used to hypothesize that the sabercats were social hunters, too. The basic scenario is that gregarious groups of the wolves, if not sabercats, were drawn to the rotting flesh in the asphalt seeps and multiple members of the group were trapped as they tried to snag an easy meal. (There is a brilliant museum display in which you can try to pull a small metal plunger out of the sort of black goo which trapped the prehistoric animals – it’s fucking difficult!) Cooperating as a group became a liability in such a sticky situation. The comparatively low prevalence of Panthera atrox may therefore indicate that these cats were either solitary or worked in smaller groups. The fact that more of the La Brea Panthera atrox specimens have been identified as males than females would seem to support this. Modern lion prides are made up of more females than males, and if Panthera atrox lived in a similar system it would be expected that more females would be found in the fossil deposits. Alternatively, I wonder if the asphalt seeps most often claimed lone males or small coalitions which had not yet established themselves in prides or had been kicked out – individuals who had to scavenge more frequently as they lacked the support of a social group. Testing such ideas is difficult, however, and the reason why Panthera atrox is so rare at La Brea remains an open question.

When I stood starting at the reconstructed skeleton of Panthera atrox at the Page Museum, I kept trying to imagine what the formidable carnivore would have looked like when properly attired in muscle, fat, fascia, fur, and the other accoutrements of a living creature. What would it have been like to see such a predator stalking along the landscape of a California denuded of office buildings and free of smog? Not much time separated the living creature from me – a paltry few thousand years, and, like everyone else, my own prehistoric forebears lived alongside such powerful predators. Yet, despite my chronological proximity to those chocolate-colored bones, there is so much we don’t know about how Panthera atrox lived. Of all the prehistoric landscapes that have come and gone during the history of this planet, the wonderful world of the Pleistocene is tantalizingly and infuriatingly close to our own.

Top Image: A composite skeleton of Panthera atrox at the Page Museum in Los Angeles, California. Photo by the author.